Risk of H5N1 B3.13, currently circulating in the US
A recent EFSA scientific assessment reviews the emergence and spread of the highly pathogenic avian influenza (HPAI) H5N1 genotype B3.13, focusing on its outbreak in U.S. dairy cattle and evaluating the potential risk of introduction into the European Union (EU).
Between March 2024 and May 2025, the virus affected 981 dairy farms across 16 U.S. states, with most cases reported in California. The rapid spread was likely driven by cattle movements, shared equipment, and contact with wild and domestic animals. Infected cattle showed mostly mild symptoms, including decreased milk production, fever, and digestive issues, with low morbidity and mortality.
The U.S. response included strict movement controls, mandatory testing, quarantine, and improved biosecurity. EFSA’s report highlights the potential for virus entry into the EU via trade in live cattle, meat, milk, and dairy products. While imports of live animals and meat are regulated, risks remain with raw or lightly processed dairy products due to the virus's resilience.
The report also emphasizes the role of migratory waterbirds in cross-continental transmission and notes uncertainties regarding viral presence in animal tissues and the effectiveness of processing methods, complicating risk assessments further.
